Access to devices / appliances on both private & guest SSID

Running Merlin-John [Fork] (version 374.43_2-29L2j9527) on an RT-AC68U and want a specific Chromecast device, Sonos speakers, and a media server to be accessible simultaneously by both guest and private WiFi networks, but where guest clients cannot access computers, tablets, phones, etc. connected to the private WiFi network. Only two settings in GUI are "Access Intranet" and "Set AP Isolated". Neither, I believe, are adequate for my needs. Suggestions sought.